I found a bug, but maybe it's not a jdownloader thing, more a bug in rocketdock oder java itself. But a workaround could help.

Rocketdock is an alternative taskbar. all your active processes and often used ones are there. blabla, you know what i mean.
When using Rocketdock [**External links are only visible to Support Staff****External links are only visible to Support Staff**
JD [beta 0.5.547] is always using 100& CPU and freezes the system for some seconds when i select JD from the "active-processes-bar" [not the all-my-tools-bar]. after moving the mousecoursor OFF rocketdock, the cpu goes down to 1%. i'm using rocketdock and a lot of tools since years, but never experienced something similar [but jd is my only one java tool!]

by the way, die cpu usage is done by jd, not rocketdock !
rocketdock is pure c++, no java.